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"Clearing the neighborhood" is an informal description of part of the process of planet formation. The phrase may be derived from a paper presented to the general assembly of the International Astronomical Union in 2000 by Alan Stern and Harold Levison. The authors used several similar phrases as they developed a theoretical basis for determining if an object orbiting a star is likely to "clear its neighboring region" of planetesimals, based on the object's mass and its orbital period.[1] The concept was used by the IAU in its August 24, 2006, redefinition of the term planet as the criterion differentiating a planet from a dwarf planet: A planet is a body with sufficient mass to have "cleared the neighbourhood around its orbit."[2] The IAU also explicitly stated that Pluto will now be considered a dwarf planet. Pluto has not cleared the neighborhood of its orbit (vis-à-vis Kuiper Belt Objects such as the Plutinos). The IAU's definition did not specify how this factor was to be measured in the general case, beyond that future decisions over borderline objects were to be made by "an IAU process," presumably in committee.

Clearly distinguishing "planets" from "dwarf planets" and other minor planets had become necessary because the IAU had adopted different rules for naming newly discovered major planets and newly discovered minor planets, without establishing a basis for telling them apart. The naming process for Eris stalled after the announcement of its discovery in 2005, pending clarification of this first step.

Details

The phrase refers to an orbiting body (a planet or protoplanet) "sweeping out" its orbital region over time, by gravitationally interacting with smaller bodies nearby. Over many orbital cycles, a large body will tend to cause small bodies either to accrete with it, or to be disturbed to another orbit. As a consequence it does not then share its orbital region with other bodies of significant size, except for its own satellites, or other bodies governed by its own gravitational influence. This latter restriction excludes objects whose orbits may cross but which will never collide with each other due to orbital resonance, such as Jupiter and the Trojan asteroids, Earth and 3753 Cruithne or Neptune and the Plutinos.[1]

Steven Soter of the Department of Astrophysics, American Museum of Natural History, has written that "A heliocentric body with Λ > 1 [viz., a planet] has cleared a substantial fraction of small bodies out of its orbital neighborhood."[3] Λ (Lambda) is a parameter proposed by Stern and Levison[1] that measures the extent to which a body scatters smaller masses out of its orbital zone over a long period of time. Mathematically Λ is defined as

$\Lambda = \frac{kM^2}{P}$

where k is approximately constant and M and P are the scattering body's mass and orbital period, respectively. Two bodies are defined to share an orbital zone if their orbits cross a common radial distance from the primary, and their non-resonant periods differ by less than an order of magnitude. The order-of-magnitude similarity in period requirement excludes comets from the calculation, but the combined mass of the comets turn out to be negligible compared to the other small solar system bodies anyway so their inclusion would have little impact on the results. Stern and Levison found a gap of five orders of magnitude in Λ between the smallest terrestrial planets and the largest asteroids and Kuiper Belt Objects (KBOs).

Soter went on to propose a parameter he called the "planetary discriminant", designated with the symbol µ (mu) , that represents an experimental measure of the actual degree of cleanliness of the orbital zone. µ is calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone.

The calculated parameters for major solar system bodies are:[3]

Body Earth masses Λ/ΛE* µ**
Mercury 5.5×10−2 1.26×10−2 9.1×104
Venus 8.15×10−1 1.08 1.35×106
Earth 1.00 1.00 1.7×106
Mars 1.07×10−1 6.1×10−3 1.8×105
Ceres 1.5×10−4 8.7×10−9 0.33
Jupiter 3.177×102 8.51×103 6.25×105
Saturn 95.2 3.08×102 1.9×105
Uranus 14.56 2.51 2.9×104
Neptune 17.1 1.79 2.4×104
Pluto 2.2×10−3 1.95×10−8 0.077
Eris 5×10−3 3.5×10−8 0.10

*Λ/ΛE = M2/P, in Earth masses squared per year.
**µ = M/m, where M is the mass of the body, and m is the aggregate mass of all the other bodies that share its orbital zone.

Controversy

Stern, currently leading the NASA New Horizons mission to Pluto, objects to the reclassification of Pluto on the basis that—like Pluto—Earth, Mars, Jupiter and Neptune have not cleared their orbital neighbourhoods either. Earth co-orbits with 10,000 near-Earth asteroids, and Jupiter has 100,000 Trojan asteroids in its orbital path. "If Neptune had cleared its zone, Pluto wouldn't be there," he now says.[4]

However, in 2000 Stern himself wrote, "we define an überplanet as a planetary body in orbit about a star that is dynamically important enough to have cleared its neighboring planetesimals ..." and a few paragraphs later, "From a dynamical standpoint, our solar system clearly contains 8 überplanets"—including Earth, Mars, Jupiter, and Neptune.[1] Most planetary scientists understand "clearing the neighborhood" to refer to an object being the dominant mass in its vicinity, for instance Earth being many times more massive than all of the NEA's combined, and Neptune "dwarfing" Pluto and the rest of the KBO's.[3]

Stern and Levison's paper shows that it's possible to estimate whether an object is likely to dominate its neighborhood given only the object's mass and orbital period, known values even for extrasolar planets. In any case, the recent IAU definition specifically limits itself only to objects orbiting the Sun.[2]